Hubble’s most remarkable and important discovery came soon after his establishing that many of the nebulae were extragalactic systems. He found a systematic increase in the redshift of their spectral lines with increasing distance. Adopting the most straight¬forward interpretation of the redshift as being a Doppler shift due to the recession of the source, we can see an immediate interpretation of Bubble’s result: the Universe is expanding, and the most distant galaxies are rushing away the fastest. Taken at face value, we are led by this discovery to the conclusion that the Universe was denser in the past than now, and we may even speculate that there may have been a time in our past when all the matter in the Universe was highly compressed. Such are the possible ramifications of Bubble’s discovery, a discovery that surely ranks as one of the outstanding achievements of modern physics. Of course, we should proceed with care. It is possible that the simple interpretation of the redshift is not correct, and that the expansion is illusory. Even if we accept, the fact of expansion, it does not necessarily follow that the*, Universe was denser in the past than now, for implicit in that conclusion is the assumption that matter in the Universe is neither created nor destroyed. It is the task of the astrophysicist to put those ideas to the test, to distinguish clearly what is well-established, and perhaps that some ideas are, a priori, more plausible than others. So, with regard to the redshift, the hypothesis that it is a Doppler shift due to the RECESSION OF THE GALAXIES is scientifically acceptable: it is consistent with the laws of physics as we know them. No other scientifically acceptable hypothesis has yet been proposed. On the other hand, we have no proof that this is the explanation. We take the simplest course and assume that the cosmic expansion is real. We can do a lot better as regards supporting the contention that the Universe was denser in the past, and we can moreover provide strong evidence that it was also much hotter.

Before presenting this further evidence for the HOT BIG BANG THEORY, it should be remarked that in the time since Bubble’s discovery of the expansion, much more data has become available which provides striking confirmation of Bubble’s linear relationship between recession velocity and distance. The BUBBLE LAW, as it is now called, is known to extend a distance a hundred times that investigated by Hubble, and there is no obvious deviation from a simple-straight line relationship :

Velocity = Bubble’s parameter X distance in Mpc

From the modern data, the Bubble parameter is found to be 75 kilometers per second per megaparsec with a possible uncertainty of 25km/sec Mpc. That is, for every extra megaparsec away in distance from us. the galaxies recede faster by 75 kilometers per second. This is the present RATE OF EXPANSION OF THE UNIVERSE. It should also be noted that because light travels at a finite speed, the more distant galaxies are being observed as they were a consider¬able time in our past. There is therefore the possibility of inferring the nature of the cosmic expansion at much earlier times.

There is an interesting side-issue relating to the cosmic expansion and the redshifting of the light from distant galaxies: this is the so-called OLBERS’ PARADOX, which may be set forth as follows. If the Universe were infinite, yet not expanding, every line of sight from Earth would eventually intersect the surface of a star. In that case, the sky would be everywhere as bright as the surface of a typical star, in other words, as bright as the Sun’s surface! Olbers recognized the darkness of the night sky as an important paradox. Now we can understand the resolution of the paradox in terms of known facts about the Universe. Stars which lie in galaxies at great distances from us have their light redshifted out of the optical wave-band and into the infrared or beyond. Thus they do not contribute I o the optically visible light of the night sky and that is why the sky is dark at night.

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